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e45e4b3cda
we shouldn't call signal.disconnect in __del__ because it can lead to
deadlocks in Django signal dispatch code
The Signal.connect, Signal.disconnect, and Signal._live_receivers
methods all share a threading.Lock():
22a60f8d0b/django/dispatch/dispatcher.py (L49)
It's possible for this to lead to a deadlock:
1. Have code that calls Signal._live_receivers and enter the critical
path inside the shared threading.Lock()
2. Python garbage collection occurs and finds one or more LDAPBackend
objects with no more references
3. This __del__ is called, which calls Signal.disconnect
4. Code in Signal._disconnect attempts to obtain the (already held)
threading.Lock
5. Python hangs forever while attempting to garbage collect